According to Tim Noakes, emeritus professor of exercise and sports science at the University of Cape Town, mild exercise enhances immunity in at least three ways: by prompting the immune system to release a protein which increases the body's temperature, making it hostile to invading bugs; by producing more interferon, a protein that is active against viruses; and by increasing the activity of natural killer white blood cells, which have anti-tumour and antiviral potential.

A review in the New England Journal of Medicine states that natural killer cells are currently undergoing evaluation as anti-tumour agents in several clinical trials: "Regular exercise, including daily brisk walking, is associated with a lower risk of several cancers and with lower risks of tumour recurrence and death among survivors, particularly of colorectal and breast cancers."

Commenting on recent research, consultant oncologist Seamus O'Reilly of Cork University Hospital told The Irish Times: "Studies in animal models of cancer have shown that muscle exercise increases anti-cancer immune activity. This is now being tested in clinical trials and supports observations that exercise may reduce cancer recurrence in patients with breast and colon cancer.

"Exercise is also an important component in weight control," he said. "Excess weight is responsible for 15 per cent of the cancers we see in Ireland, a figure that we expect will increase in the years ahead due to the obesity epidemic."

Body stress

Still, exercise can be a double-edged sword: Moderate exercise confers health benefits but excessive exercise can stress the body.

For example, a 24-year-old elite male triathlete completed a gruelling five-hour training session and became feverish with a very high temperature. A fortnight later he entered an Ironman race but developed diarrhoea, walking to the finish. The triathlete was diagnosed with bacterial pneumonia, treated with antibiotics, stopped all exercise for eight weeks and recovered.

He was treated by Dr Danica Bonello-Spiteri, a consultant in sports and exercise medicine (sportsmedicinemalta.com) as well as an elite triathlete.

“High training loads can suppress the body’s immune system and increase the risk of an athlete contracting an upper respiratory tract infection,” or URTI, she said. “A combination of stress – both physiological and psychological – and poor sleep can disrupt an individual’s resistance to infection. Elite athletes not only push their bodies to extremes, but they often postpone seeking medical help.”

According to the British Association of Sport and Exercise Sciences, “Acute exercise bouts temporarily depress immune function and can last three to 24 hours after exercise, depending on its intensity and duration.

“The precise nature of so-called URTIs in high-performance athletes is unclear: they could be caused by microbes such as bacteria or viruses or they may be associated with exercise-associated inflammatory processes. Immune depression after prolonged exercise may be related to increased concentrations of stress hormones [eg adrenaline and cortisol] in the bloodstream.”

So what practical steps can runners and other exercisers take to safeguard their immune systems? First on Dr Bonello-Spiteri’s list is sufficient sleep. “I recommend at least seven hours a night, allowing the body to rest and to undertake repair. This includes warding off infections. Diet is also important, so eat fresh, homemade food and avoid anything that is processed, canned, or artificially made in factories.”

Cooling down

She also says that if you train in the rain, remember: as soon as you stop exercising, the body will continue to keep on cooling down. “So it’s vital to change out of your wet training clothes as soon as you can, shower, change into dry clothes and warm up.”

But perhaps her most important advice is: “Learn to listen to your body . . . if you’re feeling run down from a cold, don’t feel that you must stick to your hard-training programme at all costs. Either reduce training duration and/or intensity or take a rest day, including an extra nap if needed.”

Does training volume or intensity adversely affect immunity? Last year a team led by Neil Walsh, director of the Extremes Group at Bangor University, reported on how the immune system was affected by both training intensity and duration. Walsh said: "Our results challenge the idea that immunity is suppressed by short-duration, high-intensity exercise."

The researchers determined the overall immune responses in 64 healthy males who ran at different running intensities and durations. Those undertaking prolonged, moderate-intensity running – two hours at 60 per cent of their aerobic capacity – showed impaired immunity, but not after short-duration, high-intensity running – 30 minutes at 80 per cent of their aerobic capacity.

“It appears,” Walsh told me, “that exercise lasting two hours or more decreases the immune response.”

Immune boosting

As for immunity and nutrition, Dr Glen Davison, senior lecturer and researcher in exercise physiology at the University of Kent, said: "There are many nutritional 'products' on the market, or dietary practices that are advocated as 'immune boosting' etc. But many are not backed up by credible scientific evidence."

Running is indeed healthy, but your training should include sufficient rest periods to allow the body’s immune system to recover. Consider the duration/intensity of your training. Sleep and a healthy diet enhance immunity, but the evidence for the benefits of nutritional supplements is debatable.